Gradient magnetic field electronic power supply for gradient coil

ABSTRACT

A gradient magnetic field power supply includes an amplifier for supplying current to a gradient coil. Between input and output of the amplifier is connected a feedback circuit which feeds a portion of an output current at the output back to the input and has a built-in phase compensating circuit for compensating the phase of the feedback current. The frequency response of the phase compensating circuit is made variable and adjusted to fit variations in the load impedance of the amplifier with time. This ensures optimum phase compensation.

This application is a Continuation of application Ser. No. 08/612,579,filed on Mar. 8, 1996, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a gradient magnetic field power supplywhich supplies a current to a gradient coil for producing a gradientmagnetic field.

2. Description of the Related Art

With very high speed imaging methods, such as echo planar imaging (EPI),the acquisition of data needed to reconstruct one magnetic resonanceimage is finished in some tens of milliseconds. In order to implementvary high speed imaging, it is required to cause a gradient magneticfield to rise rapidly. A general gradient coil has an inductance of somemillihenries and a resistance on the order of one ohm to hundreds ofmilliohms. In the ordinary imaging method, a current of 100 to 200amperes is used to generate a gradient magnetic field of about 1 G/cm.To provide a current of 150 amperes and a coil inductance of onemillihenry within one millisecond, a supply voltage of 150 volts arerequired, assuming that there is no resistance component involved. Inthe very high-speed imaging method, it is necessary to generate acurrent about three times greater, that is a high intensity gradientmagnetic field, within about five to ten times shorter time. Forexample, to generate a current of 450 amperes within 200 microseconds, avoltage of 2250 volts must be applied. Gradient magnetic field powersupplies generally used with magnetic resonance imaging apparatus have acapability of providing an output voltage of at most 200 volts, whichdoes not meet the above condition. Thus, various measures have beentaken so far to accomplish the fast rise of a gradient field requiredwith the very high speed imaging through the use of general gradientmagnetic field power supplies.

A first measure is to employ resonance for the purpose of raising acurrent supplied to a gradient coil. Between a power supply and agradient coil is connected a capacitor, which constitutes a resonantcircuit with the coil. Owing to resonance, a current rises rapidly. Whenthe current reaches a predetermined level, the capacitor is removed, sothat the coil is directly connected to the power supply. The powersupply then supplies a current of constant magnitude to the coil.

Note that the power supply comprises a negative feedback amplifier thefeedback circuit of which consists of a resistor and a phasecompensation capacitor.

When a resonance phenomenon is used to raise a current as describedabove, the load impedance of the power supply varies with time. Duringthe interval when the current is rising, the load impedance isdetermined by the capacitance C of the capacitor and the inductance Land the resistance R of the gradient coil. When the current is constant,the load impedance is determined by the inductance L and the resistanceR of the gradient coil.

With variations in the load impedance with time, the feedback circuitwill not achieve proper phase compensation. Therefore, the use ofresonance serves to raise the current fast on the one hand, and lowersthe stability of the power supply on the other hand.

In addition, when the resonance is employed so as to cause the currentto rise fast, the current rise time will depend on the time constant ofthe resonant circuit. It is, therefore, impossible to regulate the risetime.

A second measure is to support the main gradient field power supply withan auxiliary high-voltage power supply when the current to the gradientcoil rises.

FIG. 1 shows an arrangement of such a gradient magnetic power supply asuses an auxiliary power supply. Switches SW1 to SW4 are connected in abridge configuration. A series combination of a main power supply and agradient coil is connected between two points, each on a respective oneof the two branches of the bridge. An auxiliary power supply isconnected in series with a switch SW5 between the two other points ofthe bridge. A pair of switches SW1 and SW3 on the opposed arms of thebridge is simultaneously turned ON or OFF. Likewise, the other pair ofswitches SW2 and SW4 is simultaneously switched ON or OFF. Selectivelyturning ON one of the pairs of switches allows the polarity of currentsupplied from the auxiliary power supply to the gradient coil to bechanged arbitrarily. The switch SW5 is turned ON during the intervalwhen the current is rising or falling. The auxiliary power supplysupports the main power supply so as to allow the supply current to thegradient coil to rise fast.

With the echo planar imaging, an alternated gradient magnetic fieldwhose polarity alternates fast is needed. The waveform of a gradientmagnetic field is shaped into a sinusoidal waveform etc. To change thecurrent polarity in succession, the fast switching of the switches SW1to SW4 is required.

However, the fast switching causes switching noise, which deterioratesthe signal-to-noise ratio in magnetic resonance image data.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a gradient magnetic fieldpower supply which permits optimum phase compensation to be achievedwhile following a time-varying load impedance.

It is another object of the invention to provide a resonance type ofgradient magnetic field power supply which permits the rise time ofcurrent supplied to a gradient coil to be regulated.

It is still another object of the invention to provide anauxiliary-power-supply-based type of gradient magnetic field powersupply which permits switching noise to be reduced.

According to an aspect of the invention there is provided a gradientmagnetic field power supply for gradient coil comprising: amplifiermeans for supplying a current to the gradient coil; feedback circuitmeans for feeding a portion of an output current of the amplifier meansback to an input of the amplifier; phase compensating means forcompensating the phase of a feedback current from the feedback circuitmeans to the amplifier means, the phase compensating means having itsfrequency response made variable; and changing means for changing thefrequency response of the phase compensating means according to a changein a load impedance of the amplifier means.

According to another aspect of the invention there is provided agradient magnetic field power supply for gradient coil comprising:amplifier means for supplying a current to the gradient coil; acapacitor; and current raising means for causing the current to rise byswitching between first and second states, the first state being suchthat the capacitor is inserted between the gradient coil and theamplifier means and the gradient coil and the capacitor forms a seriesresonant circuit and the second state being such that the capacitor isremoved from between the gradient coil and the amplifier means and thegradient coil and the amplifier means are directly connected to eachother.

According to still another aspect of the invention there is provided agradient magnetic field power supply for gradient coil comprising: amain power supply for supplying a current to the gradient coil; anauxiliary power supply for supplying a current to the gradient coil;switching means for alternating the polarity of the auxiliary powersupply with respect to the gradient coil to thereby alternate thedirection of a gradient magnetic field produced by the gradient coil;and lowpass filters connected to both ends of the gradient coil.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention and, together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 shows an arrangement of a conventional gradient magnetic fieldpower supply which uses an auxiliary power supply to support a mainpower supply;

FIG. 2 is a general view of a magnetic resonance imaging apparatus;

FIG. 3 shows an arrangement of a gradient magnetic field power supplyaccording to a first embodiment of the invention;

FIG. 4 shows an arrangement of the feedback circuit of FIG. 3;

FIG. 5 shows current waveforms;

FIG. 6 shows the states of the switches and the diodes of FIG. 2 in eachof the intervals shown in FIG. 5;

FIGS. 7A, 7B and 7C each show the state of the power supply in arespective one of the intervals A, B and C shown in FIG. 5;

FIGS. 8A, 8B and 8C each show the state of the power supply in arespective one of the intervals D, E and F shown in FIG. 5;

FIG. 9 shows the load impedance of the power supply in each of theintervals of FIG. 5;

FIG. 10A is a diagram for use in explanation of the load impedance Z1;

FIG. 10B is a diagram for use in explanation of the load impedance Z2;

FIG. 11 shows current waveforms output from the power supply andvariations in load impedance of the power supply with time;

FIG. 12 shows switch states and selected capacitors in each of theintervals of FIG. 5;

FIG. 13 shows an arrangement of a gradient magnetic field power supplyaccording to a second embodiment of the invention;

FIG. 14A shows the power supply in resonance;

FIG. 14B shows the power supply in non-resonance;

FIG. 15 is a diagram for use in explanation of the principle ofregulation of the rise time of current;

FIGS. 16A, 16B and 16C show changes in current waveform resulting fromchanges of the ratio of the resonant state to the non-resonant state induration;

FIG. 17 shows changes in current rise time each corresponding to arespective one of the current waveforms of FIGS. 16A, 16B and 16C;

FIG. 18 show current waveforms based on constant rise time control;

FIG. 19 shows an arrangement of a gradient magnetic field power supplyaccording to a third embodiment of the invention;

FIG. 20 shows current waveforms with linear rise;

FIG. 21 shows the state of the gradient magnetic field power supply inthe interval when the current is rising in FIG. 20;

FIGS. 22A and 22B show current waveforms approximating sinusoidal wave;

FIG. 23 shows a first state of the gradient magnetic field power supply;and

FIG. 24 shows a second state of the gradient magnetic field powersupply.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 is a schematic representation of a magnetic resonance imagingapparatus, which includes a coil assembly as its principal component.The coil assembly has a cylindrical space within it to accommodate aportion of a human body under examination. The human body is laid downon a couch 6 and moved to within the space.

A static magnetic field magnet 1 is supplied with current from a staticmagnetic field power supply 2 to provide a static magnetic field withinthe space, when the superconducting magnet is excited. Note here thatthe direction of the static magnetic field is defined as the Z-directionof the conventional XYZ coordinate system, a direction perpendicular tothe Z-direction is defined as the X-direction, and a directionperpendicular to the XZ plane is defined as the Y-direction.

A gradient coil assembly 3 is supplied with a current from gradientmagnetic field power supplies 4 to provide gradient fields Gx, Gy, andGz. The gradient field Gx is one whose strength varies linearly alongthe X-direction, Gy is one whose strength varies linearly along theY-direction, and Gz is one whose strength varies linearly along theZ-direction.

A transmitting probe 7, also called an RF coil, is supplied withradio-frequency current from a transmitter 8 to provide aradio-frequency magnetic field (an RF pulse), which excites magneticspins of, for example, protons within the human body 5. The excitedmagnetic spins produce a magnetic resonance signal, which is received bya receiving probe 9 and then applied to a receiver 10. A single probemay be used as the transmitting probe 7 and the receiving probe 9.

The receiver 10 amplifies and detects the magnetic resonance signal,which, in turn, is converted by a data collector 12 into digital form.

A computer 13 reconstructs magnetic resonance image data from digitalsignals output from the data collector 12. A typical example of thereconstruction processing is two-dimensional Fourier transform. An imagedisplay 15 visually displays a magnetic resonance image reconstructed bythe computer 13. A console 14 is connected to the computer 13.

A system controller 11 sequentially controls the gradient magnetic fieldpower supply 4, the transmitter 8, the receiver 10, and the datacollector 12 to carry out a pulse sequence, for example, a spin-echopulse sequence.

FIG. 3 shows an arrangement of the gradient magnetic field power supply4, which is constructed from an amplifier 401, a waveshaper 402, aswitching circuit 403, a feedback circuit 413, and a controller 414. Theamplifier 401 is connected to the coil 3 through the switching circuit403.

The switching circuit 5 includes four diodes 404 to 407 and fourhigh-speed semiconductor switches 408 to 411, such as IGBTs (InsulatedGate Bipolar Transistors). The four diodes 404 to 407 and the foursemiconductor switches 408 to 411 are connected so that switching can bemade between a first state (resonant state) and a second state(non-resonant state) according to combinations of ON/OFF states of thefour semiconductor switches. Further, the diodes 404 to 407 and thesemiconductor switches 408 to 411 are connected so that the combinationsof ON/OFF states of the four semiconductor switches allow the directionof current to be switched.

The resonant state refers to a state in which a capacitor 412 isinserted between the amplifier 401 and the gradient coil 3, in whichcase the capacitor and the gradient coil forms a resonant circuit. Thenon-resonant state refers to a state in which the amplifier and thegradient coil are directly connected to each other with no capacitorinserted.

The controller 414 performs ON/OFF control on the semiconductor switches408 to 411 so that the resonant state will be obtained during theinterval when current rises and during the interval when the currentfalls. Also, the controller controls the semiconductor switches 408 to411 to switch from the resonant state to the non-resonant state when thecurrent rises up to a predetermined level and then obtain steadycurrent. Thus, the gradient power supply 4 is a resonant type of powersupply that increases the speed at which current rises and falls throughthe use of resonance.

With this type of power supply, the load impedance of the amplifier 10varies between the resonant state and the non-resonant state. The loadimpedance (first load impedance) Z1 in the resonant state is determinedby the capacitance C of the capacitor 412 and the inductance L and theresistance R of the gradient coil 3. The load impedance (second loadimpedance) Z2 in the non-resonant state is determined by the inductanceL and the resistance R of the gradient coil 3 only.

The waveshaper 402 provides a waveform signal to the amplifier 401,which changes the amplitude of its output current according to thewaveform signal.

The amplifier 401 is of a negative feedback type that ensures operationstability. A feedback circuit 413 feeds a portion of output current ofthe amplifier 401 back to its input in the form of negative feedback.The feedback circuit 413 has a built-in phase compensation circuit toensure operation stability.

FIG. 4 shows an arrangement of the phase compensation circuit of thefeedback circuit 413. The phase compensation circuit has a first one 421and a second one 422. The first and second phase compensation circuits421 and 422 are each connected in parallel with the amplifier 401. Ahigh-speed semiconductor switch 427 is connected in series with thefirst phase compensation circuit 412, while a high-speed semiconductorswitch 427 is connected in series with the second phase compensationcircuit 422.

The semiconductor switches 427 and 428 are operated in the oppositephases under the control of the controller 414. That is, when the switch427 is closed, the switch 428 is open and vice versa. When the switch427 is closed and the switch 428 is open, the first phase compensationcircuit 421 is switched into connection with the amplifier 401 and thesecond phase compensation circuit 422 is switched out of connection withthe amplifier. When the switch 427 is open and the switch 428 is closed,on the other hand, the first compensation circuit 421 is disconnectedfrom the amplifier and the second compensation circuit 422 is connectedto the amplifier.

The first phase compensation circuit 421 has a first fixed capacitor 423and a first fixed resistor 424. The second phase compensation circuit422 has a second fixed capacitor 425 and a second fixed resistor 426.The frequency response (first frequency response) FC1 of the first phasecompensation circuit 421 is determined by the capacitance C1 of thefirst fixed capacitor 423 and the resistance R of the first fixedresistor 424. The frequency response (second frequency response) FC2 ofthe second phase compensation circuit 422 depends on the capacitance C2of the second fixed capacitor 425 and the resistance R2 of the secondfixed resistor 426. The first and second frequency responses FC1 and FC2differ from each other.

The frequency response FC1 of the first phase compensation circuit 421is determined so as to make optimum phase compensation for the firstload impedance Z1 in resonance. The frequency response FC2 of the secondphase compensation circuit 422 is determined so as to make optimum phasecompensation for the second load impedance Z2 in nonresonance.

The first load impedance Z1 is determined by the impedance of the seriesresonant circuit composed of the gradient coil 3 and the capacitor 412,while the second load impedance Z2 is determined by the impedance of thecircuit when the capacitor 412 is removed from the series resonantcircuit.

Hereinafter the operation of the switching circuit 403 will bedescribed.

FIG. 5 shows a general waveform of an output current of the gradientpower supply 4. In FIG. 5, intervals A and D are rising intervals,intervals B and E are steady-state intervals, and intervals C and F arefalling intervals. FIG. 6 shows the ON/OFF states of the switches 408 to411 and the conductive states of the diodes 404 to 407 in each of theintervals shown in FIG. 5. Each of the blanks in FIG. 6 stands for the"OFF" or "nonconductive" state of a corresponding component.

During the interval A, the switches 409 and 410 are turned ON as shownin FIG. 7A. The capacitor 412 and the gradient coil 3 are thus connectedin series, forming a series resonant circuit. In operation, electriccharge stored on the capacitor 412 is applied to the gradient coil 3 inthe form of a high voltage. A current flows in the forward direction andgradually increases in magnitude. When the current reaches apredetermined magnitude (+Ip), a transition is made from the interval Ato the interval B.

During the interval B, as shown in FIG. 7B, the switch 410 continues tobe turned OFF and the switch 409 is moved from ON to OFF. The gradientcoil 3 is thus connected to the amplifier 401 through the diode 404,resulting in the non-resonant state. During this interval B, electriccharge stored on the capacitor 412 is stored in the gradient coil asinductive energy. A current flows in the forward direction and itsmagnitude is maintained at Ip. When the interval B reaches a given time,a transition is made from the interval B to the interval C.

During the falling interval C, the switch 410 is switched from ON toOFF. The capacitor 412 and the gradient coil 3 are therefore connectedin series, resulting in the resonant state. During this interval C, theinductive energy stored in the gradient coil 3 is stored again on thecapacitor 412 in the form of electric charge. A current flows in theforward direction and gradually decreases in magnitude.

During the next rising interval D, the switches 408 and 411 are turnedON as shown in FIG. 8A. Thereby, the capacitor 412 and the gradient coil3 are connected in series, resulting in resonant state. During thisinterval D, the electric charge on the capacitor 412 is applied to thegradient coil 3. A current flows in the forward direction and graduallyincreases in magnitude. When the current reaches a given magnitude(-Ip), a transition is made from the interval D to the interval E.

During the interval E, the switch 408 continues to be ON and the switch411 is switched from ON to OFF as shown in FIG. 8B. The gradient coil 3becomes connected to the amplifier 401 through the diode 406, resultingin non-resonant state. During this interval E, the electric charge onthe capacitor 412 is stored in the gradient coil as inductive energy. Acurrent flows in the reverse direction with its magnitude maintained atIp.

During the falling interval F, as shown in FIG. 8C, the switch 408 ischanged over from ON to OFF. Thereby the capacitor 412 and the gradientcoil 3 are connected in series to form a series resonant circuit. Duringthis interval C, the inductive energy stored in the gradient coil isstored again on the capacitor 412 in the form of electric charge. Acurrent flows in the reverse direction and its magnitude graduallydecreases.

FIGS. 9 and 11 show the load impedances in the intervals shown in FIG.5. During the intervals A, C, D and F when resonance occurs, the firstload impedance Z1 is determined by the capacitance C of the capacitor412 and the inductance L and the resistance R of the gradient coil 3 asshown in FIG. 10A. During the non-resonant intervals B and E, the secondimpedance Z2 is determined by the inductance L and the resistance R ofthe gradient coil 3 as shown in FIG. 10B.

Thus, the load impedance of a resonant type of power supply changes in acycle of Z1-Z2-Z1.

Next, a selection between the first and second phase compensationcircuits will be described. The frequency response FC1 of the firstphase compensation circuit 421 is determined so as to make optimum phasecompensation for the first load impedance Z1 in resonance. The frequencyresponse FC2 of the second phase compensation circuit 422 is determinedso as to make optimum phase compensation for the second load impedanceZ2 in non-resonance.

The controller 414 selects between the first and second phasecompensation circuits 421 and 422.

FIG. 12 shows the ON/OFF states of the switches 427 and 428 and thecorresponding phase compensation circuit in each of the intervals shownin FIG. 5. During the intervals A, C, D and F when resonance isproduced, the switch 427 is turned ON and the switch 428 is turned OFF,selecting the first phase compensation circuit 421. Then, the optimumphase compensation is realized for the load impedance Z1. During theintervals B and E when no resonance is produced, on the other hand, theswitch 427 is turned OFF and the switch 428 is turned ON, selecting thesecond phase compensation circuit 422. The optimum phase compensation isrealized for the load impedance Z2.

With a resonant type of power supply, the load impedance varies withtime. By causing the frequency response of the phase compensationcircuit to follow the variations in the load impedance, the optimumphase compensation can be constantly made.

In the present embodiment, frequency response changes are made by makinga selection among multiple phase compensation circuits with differentfrequency responses, two in this example. The frequency response of eachphase compensation circuit is fixed. Thus, the frequency response can bechanged faster than with a single phase compensation circuit which has avariable capacitor and a variable resistor and has its frequencyresponse changed by tuning each of them. In addition, an error infrequency response due to variable-component malfunctions will notoccur.

Moreover, a switch is provided for each of the phase compensationcircuits and a selection is made from among the phase compensationcircuits by selectively turning the corresponding switch ON. Thisprovides a fast, reliable selection between the phase compensationcircuits.

FIG. 13 shows an arrangement of a gradient magnetic field power supplyaccording to a second embodiment of the invention. In this figure, likereference numerals are used to denote corresponding parts to those inFIG. 3 and description thereof is omitted. As with the first embodiment,the second embodiment is directed to a resonant type of power supplythat utilizes resonance for current rise. A problem with this type ofpower supply is that the rise time of current is uniquely determined bya resonance frequency and thus cannot be variable. The second embodimentis intended to provide a resonant type of power supply that permits thecurrent rise time to be variable.

FIG. 14A shows the power supply in the resonant state in which thecapacitor 412 is inserted between the amplifier 401 and the gradientcoil 3, while FIG. 14B shows the power supply in the non-resonant statein which the gradient coil 3 is directly connected to the amplifier 401.The arrangement of the switching circuit 403 and the switching controlbetween the resonant state and the non-resonant state remain unchangedfrom the first embodiment.

The controller 431 allows the switching circuit 403 to be switchedbetween the state of FIG. 14A and the state of FIG. 14B during thecurrent rising interval. During the current rising interval, therefore,the resonant state and the non-resonant state alternate.

FIG. 15 shows a curve representing the rise of a sinusoidal current whenonly the resonant state is utilized and a curve representing the rise ofa current when the resonant state and the non-resonant state alternate.In this figure, PA stands for the duration of the resonant state and PBstands for the duration of the non-resonant state.

In the resonant state, the capacitor 412 and the gradient coil 3 areconnected in series. Electric charge stored on the capacitor is appliedto the gradient coil as a high voltage. A current flows in the forwarddirection (or the reverse direction) and its magnitude graduallyincreases.

In the non-resonant state, on the other hand, the gradient coil 3 isdirectly connected to the amplifier 401. In this state, electric chargestored on the capacitor is stored in the gradient coil as inductiveenergy. A current flows in the forward direction (or the reversedirection) and its magnitude is maintained at Ip.

Thus, the rise time of current when the resonant state and thenon-resonant state alternate during the current rising interval becomeslonger than that when the resonant state is kept during the currentrising interval.

Next, the regulation of the current rise time will be described.

The controller 431 repeats a cycle consisting of the intervals PA andPB. A change of the current rise time is made by changing a ratiorelated to the length ΔTA of the interval PA and the length ΔTB of theinterval PB in the cycle. Suppose here that the ratio is given by

    ΔTA/(ΔTA+ΔTB)

Namely, the ratio is defined as the ratio of the duration of theresonant state to the period of the cycle. That is, the closer the ratiois to 1.0, the shorter the current rise time becomes and vice versa.

From the viewpoint of control, the most straightforward way to regulatethe current rise time is to make the length ΔTA of the interval TA fixedand the length ΔTB of the interval TB variable. The longer the lengthΔTB, the longer current rise time becomes and vice versa. With ΔTB=0,the current rises with resonance only and its rise time becomesshortest.

It is more preferable that the length ΔTB of the interval TB in onecycle be set to an integral multiple of a unit time ΔTunit (=n×ΔTunit).The shorter the unit time is set, the more finely the current rise timecan be variable. Both the length ΔTA of the interval TA and the unittime ΔTunit are preferably set to the shortest switching time Δt fromthe point of view of the performance of the switches 408 to 411. In thiscase, "n" is treated as a parameter for determining the current risetime.

The controller 431 has a built-in ROM 433, in which a correspondence isestablished between rise times and parameter values. Upon receipt of acontrol signal representing a required rise time from the sequencecontroller 11, the controller 431 reads the value of the parameter ncorresponding to that rise time from the ROM 433.

The controller 431 continuously supplies a first control signal to theswitching circuit 403 for the time ΔTA and then continuously supplies asecond control signal to the switching circuit for the time (n×ΔTunit)in accordance with the parameter value read from the ROM 433. Thecontroller 431 repeats this cycle.

The first control signal refers to a combination of ON/OFF signals tothe switches 408 to 411 for producing resonance and consists, as shownin FIG. 14A, of ON signals to the switches 409 and 410 and OFF signalsto the switches 408 and 411. The second control signal refers to acombination of ON/OFF signals to the switches 408 to 411 for producingnon-resonance and consists, as shown in FIG. 14B, of an ON signal to theswitch 410 and OFF signals to the switches 408, 409, and 411.

In one cycle, the resonant state is continued for ΔTA and then thenon-resonant state is continued for n×ΔTunit. Further, this cycle isrepeated. Thus, the current rise time is set to the desired one.

FIGS. 16A, 16B and 16C show current rising waveforms when n=1, n=2, andn=3, respectively. FIG. 17 shows current rising characteristicscorresponding to n=1, n=2, and n=3, respectively. When n=1, the currentrises to the target value in about 180 microseconds. When n=2, thecurrent rises to the same target value in about 380 microseconds. Whenn=3, the current rises to the same target value in about 600microseconds.

Next, a description will be made of a specific way to calculate theduration PB of the non-resonant state for achieving a desired rise timeof current. Let the shortest rise time when current is made to rise inthe non-resonant state only be represented by t. Let the maximum currentof the amplifier 401 be represented by Ip and the target current valuebe represented by Ix. Further, T is supposed to represent one-fourth ofthe period of the resonant frequency.

Then, Ix is given by

    Ix=Ip×sin((π/2T)×t)                         (1)

The resonant frequency f is calculated from the inductance L of thegradient coil 3 and the capacitance C of the capacitor 412 as

    f=1/(2×π×(L×C).sup.1/2)               (2)

Since T corresponds to one-fourth of the period of the resonantfrequency, equation (2) becomes

    1/(2×π×(L×C).sup.1/2)=1/(4×T)   (3)

Equation (3) is rewritten into

    T=(π×(L×C).sup.1/2)/2                       (4)

From equation (1) the shortest rise time t is given by

    t=((2×T)/π)×sin.sup.-1 (Ix/Ip)              (5)

Suppose here that current is completely steady during the thenon-resonant interval PB. Let the minimum switching time of the switches408 to 411 be Δt. Suppose that the length ΔTA of the interval PA and theunit time ΔTunit of the interval PB are set to the minimum switchingtime Δt. Let the number of cycles required to reach the target currentvalue Ix be N.

Then, the current rise time tr is given by

    tr=(ΔTA+(n×ΔTunit))×N=N×Δt+n×N.times.Δt                                               (6)

The shortest rise time t is given by

    t=N×Δt                                         (7)

From equation (7) we obtain

    N=t/Δt                                               (8)

From equations (8) and (5) we obtain

    N=((2×T)/(π×Δt))×sin.sup.-1 (Ix/Ip)(9)

Since the current rise time is prolonged according to n, we obtain

    n×N×Δt=(tr-t)                            (10)

Thus, in order to attain the desired rise time tr, it is only requiredthat the duration ΔTB of the non-resonant state interval PB becalculated by

    ΔTB={Δt×(π-2×sin.sup.-1 (Ix/Ip))}/{2×sin.sup.-1 (Ix/Ip)}                    (11)

By setting the duration ΔTB of the non-resonant interval PB according toequation (11), the desired current rise time tr can be obtained. n isobtained from ΔTB obtained from equation (11) and the unit time Δt as

    n=ΔTB/Δt=π/(2×sin.sup.-1 (Ix/Ip))-1   (12)

Of course, the adjustment of n permits the constant rise time control asshown in FIG. 18.

In addition, a current waveform can be shaped into a desired one bychanging the ratio ΔTA/(ΔTA+ΔTB) by the controller 431.

It is possible to make an actual current waveform similar to a targetwaveform represented by a waveform signal from the waveshaper 402 moreprecisely. The controller 431 receives a waveform signal from thewaveshaper 402. The controller makes a comparison between an actualcurrent value detected through the inductance 432 and a target currentvalue represented by the waveform signal at regular intervals. When thedifference between the actual current value and the target current valueis not within a prescribed tolerance, the controller 431 increments ordecrements the parameter n to thereby extend or shorten the duration ΔTBof the non-resonant state. When that difference is greater than theupper limit of the tolerance, n is incremented to n+1 to extend theduration ΔTB of the non-resonant state. On the other hand, when thedifference is lower than the lower limit of the tolerance, n isdecremented to n-1, thereby shortening the duration ΔTB. Such feedbackcontrol permits the actual current waveform to become very close to thetarget waveform the waveform signal represents.

Note that high-frequency components produced by switching between theresonant state and the non-resonant state as in the present embodimentcan be reduced by a lowpass filter.

According to the present embodiment, as described above, even a resonanttype of gradient magnetic field power supply permits the current risetime to be variable.

A third embodiment of the invention relates to an improvement of agradient magnetic field power supply which uses an auxiliary powersupply to make current rise rapidly.

FIG. 19 shows an arrangement of a gradient magnetic field power supplyaccording to the third embodiment, which includes a main power supply 20and an auxiliary power supply 22 providing high voltage. Four switches31 to 34 are connected in a bridge configuration. The main power supply20 and a gradient coil 21 are connected in series between two points onthe two parallel branches of the bridge. The auxiliary power supply 22is connected across the two parallel branches of the bridge via a switch35. A controller 27 turns the switches 31 to 35 ON and OFF. When theswitch 35 is turned ON, the main power supply 20 and the auxiliary powersupply 22 supply current to the gradient coil 21 jointly. When theswitch 35 is turned OFF, the main power supply 20 supplies current tothe gradient coil by itself.

On either side of the gradient coil 21 there are provided lowpassfilters 23 and 24, which eliminates switching noise that occurs when theswitches 31 to 34 are operated at high speed. Switches 25 and 26 areconnected across the lowpass filters 23 and 26, respectively, and, whenturned ON, provide bypasses to remove the filters from the power supplyloop for the gradient coil. When turned OFF, the switches 25 and 26permit the lowpass filters 23 and 24 to be inserted in the power supplyloop for the gradient coil.

FIG. 20 shows current waveforms when the auxiliary power supply 22 isused to cause current to rise linearly. FIG. 21 shows the state of thepower supply when the auxiliary power supply 22 is used to cause currentto rise in the forward-current direction. To cause current to riselinearly, the switch 35 is turned ON continuously to use the auxiliarypower supply 22. To cause current to rise in the forward-currentdirection, the switches 31 and 33 on the opposed arms of the bridge areturned ON and the switches 32 and 34 on the other opposed arms areturned OFF. To make current rise in the reverse-current direction, theswitches 32 and 34 are turned ON and the switches 31 and 33 are turnedOFF. When current reaches a target magnitude, the switch 35 is turnedOFF, SW31 is turned ON, SW32 is turned OFF, SW33 is turned OFF and SW34is turned ON (SW31 is turned OFF, SW32 is turned ON, SW33 is turned ONand SW34 is turned OFF), thereby switching into independent drive by themain power supply 20. In this case, high-speed switching is unnecessary.Accordingly, the switches 25 and 26 are turned ON, so that the lowpassfilters 23 and 24 are bypassed and the directed connection between themain power supply 20 and the gradient coil 21 is obtained.

FIG. 22A shows current waveforms formed sinusoidal wave. FIG. 23 showscurrent waveforms formed sinusoidal wave that an amplitude increaseswith the time. FIG. 23 shows the state (first state) of the power supplywhen current is supplied from the auxiliary power supply 22 to thegradient coil 21 in the positive direction. FIG. 24 shows the state(second state) of the power supply when current is supplied from theauxiliary power supply 22 to the gradient coil 21 in the negativedirection. The first state shown in FIG. 23 is obtained by turning theswitches 31, 33 and 35 ON and the switches 32 and 34 OFF. The secondstate shown in FIG. 24 is obtained by turning the switches 32, 34 and 35ON and the switches 31 and 33 OFF.

In order to change a current like a sinusoidal waveform, it is requiredto alternate the current at high speed and shape its waveform. In orderto change the current at high speed, it is required to invert thepolarity of the current supplied by the auxiliary power supply 22 to thegradient coil 21 at high speed by switching between the first and secondstates at high speed. To shape the current waveform to fit a sinusoidalwaveform, it is necessary to perform pulse width modulation (PWM). Moreprecisely, the switches 31, 32, 33 and 34 need to be alternately turnedon and off, so that the switches 31, and 33 are on and the switches 32and 34 are off during a period ΔT1, and vise versa during the nextperiod ΔT2. For example, 1/(ΔT1+ΔT2) is fixed at ten times the outputfrequency or a greater value, and ΔT1/ΔT2 is changed.

In order to change a current like a sinusoidal waveform in this manner,high-speed switching of the switches 31 to 34 is necessary. Suchhigh-speed switching will produce switching noise.

During the interval when the switches 31 to 34 are switched at highspeed to obtain a current approximating a sinusoidal wave, the switches25 and 26 are turned OFF continuously as shown in FIGS. 23 and 24.Thereby, the lowpass filters 23 and 24 are operatively coupled to thegradient coil 21. As a result, switching noise is eliminated. Further,high-frequency components resulting from the intermittent turning of theswitch 35 ON and OFF can be reduced by the lowpass filters 23 and 24.

Although the preferred embodiments of the invention have been describedand disclosed, it is apparent that other embodiments and modificationsare possible.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details, and representative devices shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A gradient magnetic field power supply forgradient coil comprising:amplifier means for supplying a current to saidgradient coil; feedback circuit means for feeding a portion of an outputcurrent of said amplifier means back to an input of said amplifier;phase compensating means for compensating the phase of a feedbackcurrent from said feedback circuit means to said amplifier means, saidphase compensating means having its frequency response made variable;and changing means for changing the frequency response of said phasecompensating means according to a change in a load impedance of saidamplifier means.
 2. The power supply according to claim 1, wherein saidphase compensating means has a first phase compensating circuit with afirst frequency response and a second phase compensating circuit with asecond frequency response, and said changing means has switching meansfor making a selection between said first and second phase compensatingcircuits.
 3. The power supply according to claim 2, wherein said firstand second phase compensating circuits are connected in parallel withsaid amplifier means, and said changing means includes a first switchconnected in series with said first phase compensating circuit, a secondswitch connected in series with said second phase compensating circuit,and means for selectively turning ON one of said first and secondswitches.
 4. The power supply according to claim 1, further comprising acapacitor, and means for selecting one of first and second states, saidfirst state being such that said capacitor is inserted between saidgradient coil and said amplifier means and said gradient coil and saidcapacitor forms a series resonant circuit and said second state beingsuch that said capacitor is removed from between said gradient coil andsaid amplifier means and said gradient coil and said amplifier aredirectly connected to each other.
 5. The power supply according to claim4, wherein said phase compensating means has a first phase compensatingcircuit with a first frequency response and a second phase compensatingcircuit with a second frequency response, said first frequency responsebeing determined according to a load impedance of said amplifier meansin said first state and said second frequency response being determinedaccording to a load impedance of said amplifier means in said secondstate.